UNAIDS estimates that 14,000 new HIV-1 infections occur every day despite widespread knowledge of the protective effects of abstinence, reductions in partners, and and condom use. No vaccine candidates or topical microbicides are known to be protective, and intensive counseling was not sufficient to stop transmission completely. Novel approaches to HIV prevention warrant urgent evaluation. This is a competing continuation application for a project that aimed to understand the virological and immunological implications of daily oral antiviral use for HIV-1 prevention in seronegative persons. Since its inception 24 months ago, the project has developed an important role in the international portfolio of chemoprophylaxis research, by supporting the intensive evaluation of seroconverters for drug resistance and the storage of specimens required for virological, immunological, and pharmacological analysis. As before, the scientific aims aims of the project are to test the hypothesis that chemoprophylaxis may have durable benefits (or risks), compared with placebo, that extend beyond the period of chemoprophylactic treatment, including attenuation of the course of infection among those who become infected despite chemoprophylaxis (aim 1) due to preservation of immune responses (aim 2a) that may arise from prolonged viral antigen exposure prior to systemic infection (aim 3a). Those who remain seronegative despite continued high-level exposure to HIV-1 (despite counseling), may develop HIV-specific cell-mediated or humoral immune responses, greater natural killer cell activity, or expression of protective host restriction factors (aim 2b), which could arise from viral exposure that is contained by the antiviral drug (aim 3b). We will also determine whether drug resistant infections occur more commonly during chemoprophylaxis exposure, and whether the drug resistant viruses predominate over time even after chemoprophylaxis is stopped, which has implications for secondary transmission of drug resistant variants. Prevention trials using antiviral agents versus placebo offer unique opportunities to study the viral and host interactions that underlie transmission, establishment of the plasma viral RNA setpoint, and host protection despite exposure. This research aims to elucidate the mechanisms of chemoprophylactic success or failure, which is essential for predicting longer term benefits (and risks) and for designing the next generation of chemoprophylactic trials.